To add a watermark to your PowerPoint slides, follow these steps:

1. Open the PowerPoint presentation in PowerPoint.
2. From the Design tab, select Background Styles.
3. Select Fill Effects.
4. From the Fill Effects window, select the Picture tab.
5. Click the button labeled Select Picture.
6. Choose the jpg or png file containing the watermark.
7. Click Insert to add the watermark to your presentation.